292 THE OSTEOLOGY OF ELOTHERIUM. 
by the immense weight and length of the head. Among recent artiodactyls Hippopotamus 
has cervical vertebrae most like those of Hlotheriwm, though there are many differences 
in the details of construction. The most apparent of these differences lies in the greater 
and more uniform height and thickness of the neural spines in the modern genus. 
Doubtless the even more exaggerated massiyeness of the skull in the latter is the occasion 
of this increased development of the ceryical spines. In Sus the perforation of the neural 
arches for the passage of the spinal nerves constitutes an important difference from 
Hlothervum. 
The thoracic vertebra would appear to haye numbered thirteen, though this point 
cannot, as yet, be determined with entire certainty, and while the thoraco-lumbar vertebree 
were, in all probability, nineteen in number, as is well-nigh universal among the artio- 
dactyls, yet there were doubtless variations in the number of ribs, as is very frequently the 
case among existing animals. 
The first thoracic has a rather small centrum, with decidedly convex anterior and 
nearly flat posterior face; the facets for the rib-heads are very large and deeply 
concave. ‘The transverse process is rather short, but very large, heavy and rugose, and 
bears an unusually large, concave facet for the tubercle of the first rib. The prezyga- 
pophyses are of the cervical type, but present more obliquely inward than in the vertebree 
of the neck, while the postzygapophyses are, as in the other thoracics, placed upon the 
ventral side of the neural arch. The neural canal is high and narrow and its anterior 
opening has assumed a cordate outline. The neural spine is inclined strongly backward, 
much more so than that of the seventh ceryical, and though laterally compressed it is 
extremely high, broad and massive, greatly exceeding in all its dimensions that of the 
last neck vertebra. 
The anterior six thoracic yertebree (see Pl. XVIII, Fig. 5) are very much alike in 
appearance. The first three have broader and more depressed centra, which in the others 
become deeper vertically and more trihedral in section. The transverse processes are 
very large and prominent and carry large, deeply concave facets for the rib tubercles. 
The neural spines are very high, thick and heavy, and are strongly inclined backward, 
with club-shaped thickenings at the tips. At the seventh thoracic begins a rapid reduc- 
tion in the length and weight of the spines, a process which reaches its culmination on 
the eleventh vertebra, which has a remarkably short, weak and slender spine. This 
arrangement results in a great hump at the shoulders, somewhat as in Zitanotherium, 
though in a less exaggerated form. In both genera, the length of the anterior thoracic 
spines should be correlated with the great elongation and weight of the skull which 
requires immense muscular strength in the neck and shoulders. Hippopotamus has no 
such hump, but this is probably explaimed by its largely aquatic habits, 
